Intra-individual comparison of magnesium citrate and sodium phosphate for bowel preparation at CT colonography: automated volumetric analysis of residual fluid for quality assessment

Abstract

Aim: To perform an objective, intra-individual comparison of residual colonic fluid volume and attenuation associated with the current front-line laxative magnesium citrate (MgC) versus the former front-line laxative sodium phosphate (NaP) at CT colonography (CTC).

Materials and methods: This retrospective Health Insurance and Portability and Accountability Act-compliant study had institutional review board approval; informed consent was waived. The study cohort included 250 asymptomatic adults (mean age at index 56.1 years; 124 male/126 female) who underwent CTC screening twice over a 5 year interval. Colon catharsis at initial and follow-up screening employed single-dose NaP and double-dose MgC, respectively, allowing for intra-patient comparison. Automated volumetric analysis of residual colonic fluid volume and attenuation was performed on all 500 CTC studies. Colonic fluid volume <200 ml and mean attenuation between 300-900 HU were considered optimal. Paired t-test and McNemar's test were used to compare differences.

Results: Residual fluid volumes <200 ml were recorded in 192 examinations (76.8%) following MgC and in 204 examinations (81.6%) following NaP (p = 0.23). The mean total residual fluid volume was 155 ± 114 ml for MgC and 143 ± 100 ml for NaP (p = 0.01). The attenuation range of 300-900 HU was significantly more frequent for MgC (n = 220, 88%) than for NaP (n = 127, 50.8%; p < 0.001). Mean fluid attenuation was significantly lower for MgC (700 ± 165 HU) than for NaP (878 ± 155 HU; p < 0.001). Concomitant presence of both optimal fluid volume and attenuation was significantly more frequent for MgC 65.2% than for NaP (38%; p < 0.001).

Conclusions: Objective intra-individual comparison using automated volumetric analysis suggests that the replacement of NaP by MgC as the front-line laxative for CTC has not compromised overall examination quality.

Figure 1

Intra-individual, objective comparison of volume and attenuation of residual colonic fluid at CTC following NaP and MgC regimen using automated QA software. (a) Three-dimensional map of the correctly segmented colon (left) following the NaP regimen in a 55-year-old man. Automatically calculated total colonic residual fluid and average attenuation were 111 ml and 979 HU, respectively. (b) Three-dimensional map of the correctly segmented colon (left) following MgC regimen in the same man 6 years later at the age of 61 years. Automatically calculated total colonic residual fluid and average attenuation were 139 ml and 719 HU, respectively. Two-dimensional axial CTC images before (middle) and after (right) automated digital subtraction of residual colonic fluid allowed verification of correct capture. Note that not only large fluid collections (arrows) are captured but also smallest fluid residues (arrowheads).

Figure 2

Residual colonic fluid volume following the NaP regimen versus the MgC regimen in all 250 patients. Box and whisker plots show the median (thick line) and the box spans the interquartile range (IQR; Q1 to Q3); the whiskers extend to the most extreme observation within 1.5 times the IQR, and observations beyond that range are plotted individually and could be considered as outliers. To give a better idea of the distribution of all individual measurements, these are also shown (red for MgC and blue for NaP). Automatically quantified mean residual colonic fluid volume was higher for MgC regimen (155±114 ml) than for NaP regimen (143±100 ml) (p=0.011).

Figure 3

Residual colonic fluid attenuation following the NaP regimen versus the MgC regimen in all 250 patients. Box and whisker plots show the median (thick line) and the box spans the interquartile range (IQR; Q1 to Q3); the whiskers extend to the most extreme observation within 1.5 times the IQR, and observations beyond that range are plotted individually. To give a better idea of the distribution of all individual measurements, these are also shown (red for MgC and blue for NaP). Automatically quantified attenuation of residual colonic fluid was significantly lower for the MgC regimen (700±165 HU) than for the NaP regimen (878±155 HU; p<0.001).

Figure 4

Scatter plots illustrate the examinations that are optimal in terms of both volume (<200 ml, shaded horizontal box) and attenuation (300–900 HU, shaded vertical box) in the lower left quadrant. Simultaneously optimal fluid volume and attenuation was observed following (a) the MgC regimen in 163 examinations (65.2%) and following (b) the NaP regimen in 95 examinations (38%; p<0.001). There is an inverse correlation between volume and attenuation of residual colonic fluid for both regimens. This negative relationship is stronger for MgC (r=−0.444, p<0.001) than for NaP (r=−0.243, p<0.001).